Conventionally, aluminum alloy is extruded through a die P1 as shown in FIG. 10A, to form a hollow section of aluminum alloy. The die P1 is a port-hole die provided with a die mandrel P2 shown by a solid line in FIG. 10B for forming a hole in the hollow section. A die cap P3 shown by a two-dot dashed line is provided,in combination with the die mandrel P2, for forming the peripheral portion of the hollow section.
A mandrel P5 projects from a mandrel support P4 in the center of the die mandrel P2 of the die P1 for forming the hole in the hollow section. The die cap P3 has an inner surface P6 of an extruding hole for forming the peripheral portion of the hollow section. When the die mandrel P2 is assembled with the die cap P3, an extruding orifice P7 is defined by the tip of mandrel P5 and the inner surface P6. By forcing the extruding material through the orifice P7, the hollow section is extruded.
When the hollow section of aluminum alloy is extruded from the die P1, an output of at least 10 tons per die is generally achieved. When 7000 series Al--Zn--Mg alloy (according to Japanese Industrial Standards) or other zinc-containing aluminum alloy is extruded, however, the die P1 provides an output of less than 1 ton per die. Moreover, when extruding a zinc alloy, the conventional die P1 has a short useful life and provides little productivity as a tool.
The conventional die P1 partially cracks during the extrusion. Bridges P8 shown in FIG. 10A adjacent to the root portion "a" of support P4 of mandrel P5 are especially easily cracked due to the stress concentrations at the root portion "a" shown in FIG. 10B under the extruding load.
Others have attempted to reduce the concentrated stress by modifying the structure of the die P1. However, this merely complicates the structure of die P1 and increases the difficulty and expense in processing and assembling the die. Since increased precision in the dimensions and configuration of the extruded section is continually demanded by the industry, modifications of the die structure is not a satisfactory solution for providing the necessary reduction in concentrated stress. Furthermore, as the structure changes, the stress concentration merely shifts to another portion of the die. Consequently, cracks are simply shifted to another portion of the die, and no cracks are eliminated.